Electrical communication circuits



s. BALLANTINE ELECTRICAL COMMUNICATION CIRCUITS Filed Jan. 9. 1925 Patented 6,

UNITED-STATES PATENT OFFICE,"

STUART BALLANTINE, OF BOONTON, NEW JERSEY, ASSIGNOR 'I'O RADIO FREQUENCY LABORATORIES, INC., 01 'BOONTON, NEW JERSEY, .A. CORPORATION OF NEW JERSEY.

Application fihidIanuary 9, 1923. Serial No. 611.64 7.

This invention relates to electrical communication circuits, and'particularly to circuits which may include'one or more amplifying audion tubes.

t It is well known that the audion tubes which are employed in telephone repeating circuits and in radiotelephone transmission and receiving circuits do not give an exact enlarged reproduction of the wave impressed in upon the grid of the audion. The distortion arises from the fact that the volt-ampere characteristic of audion tubesv is not linear, but is more or less curved throughout its entire extent. When an audion is employed as it an amplifier the distortion produced is comparatively small when the operating point is located on the linear part of the curve, but some distortion is always present. When used as a detector however, the operating i point of the au-dion is on the curved portion of the characteristic, and distortion always results, since the amplitude of the rectified current varies approximately as the square of the amplitude of'ithe impressed voltage.

This invention has for an object the provision of an electrical communication circuit which may include one or more amplifying; audions in "which the distortion produced by the audion or audions is reduced or elimiao nated, such reduction or elimination of the distortion being often referred to as neutrah icing the distortion.- A further object is to provide a circuit of the type stated, in which the amplification ratio of one or more of the at; audions is varied to produce a correction.

which will reduce or eliminate the distortion normally present in such circuit. .lldore specifically an objiect ofthe invention is to pro vide a radio receiving circuit including an audion amplifier and a detector, in which the amplifying ratio of the audion is controlled to introduce a correction which will reduce or eliminate the distortion produced by the detector. a The accompanying drawing illustrates an embodiment of my invention as incorporated in a radio receiving circuit.

in the circuit illustrated, the arrangement of thedifierent elements correspondain genll l eral, to known circuits which employ audions as simultaneous amplifiers of radio and audio frequency oscillations. Thecircuit in- ELECTRICAL COMMUNICATION CIRCUITS.

teeter 5, where it is rectified. The resulting.

' HEISSUED v cludes a suitable tuned receiving circuit 1, an amplifying aud1on 2, a vario-transformer 3 .-such as described in my copending application, Ser. No. 590,514, filed Sept. 25, 1922,

a grid condenser and leak 4, and a' detector audion 5. The plate of the detector is connected to the primary side of an audio frequency transformer 6 and the secondary of the transformer 6 is connected through the tuned circuit 1 to the grid of the amplifying audion 2. The connection between the low side of the secondary'winding ofthe transformer 6 and the amplifying au'dion filament includes a G-battery 7 having appropriate means for var ing the biasing voltage im pressed upon t e grid of the audion. The telephones 8., preferably shunted by a fixed condenser l), are connected between the B battery 10 of the amplifier audion and the negative side of the A battery 11. lhe plate circuit of the detector includes the usual 15 battery 12 between the audio frequency transformer and the detector filament. Gun densers l3, 14 across the audio transformer primary and B battery 12 and -across the audio transformer secondary, respectively, provide means for lay-passing radio frequency oscillations around the transformer 6. go It will be observed that the circuits established provide for the simultaneous amplification of radio and audio frequency oscilla tions in the audion 2, but this feature is not essential to my invention, as the telephones may be included in the plate circuit of the detector. The arrangement as illustrated is preferable however, since itpermits the use a of an audio transformer of the usual con struct-ion, and the full audio frequency drop in the plate circuit occurs across the transformer,

lln operation, the voltage of the (1 battery is adjusted to bias the amplifying audion to bring the operating point upon a curved poren tion of the volt-ampere characteristic. When a modulated radio frequency wave is received, the current is amplified by the audion 2, passed through the vario-transformer 3 to the-delid w d audio currents pass-into the audio-transformer 6 and cause a second audio frequency bias voltage to be impressed upon the amplifying audion 2 in phase with, or'exactly opposite in phase to, the modulated radio frequency voltage which this audion is passing through to the detector. Since the detector operates according to the square law, i. e., the detected" audio frequency is proportional, roughly, to the square of the amplitude of the envelop of the radio frequency wave,,the peaks in this envelop are unduly extended in the audio wave, producing an alteration in the quality of the audio tone.' Due to the fact that the constant bias from the C battery has brought I limited to a radio receiving circuit of the type shown since a correcting bias voltage of substantially the same wave form as an audio frequencywave or the modulated envelop of a radio frequency wave may be applied to an audion of other amplifying circuits for the purpose of reducing or eliminating the distortion 'normallv present in such a circuit. The correcting bias may be introduced in telephone repeating circuits, in radiophone transmitting circuits, or in audio frequency amplifyingcircuits, and the bias may be impressed upon one or more of the audions of the circuit. In radio telephone reception, the invention is not limited to circuits employing an audion detector, but is useful with any detector which produces a distorted audio wave, i. e. one which does not have the same form as the envelop of the radio frequency wave.

I claim:

1. In an electrical communication circuit, an element producing a distortion in the shape of the wave traversing said circuit, an amplifying audion preceding said element, and

means for varying the amplifying ratio of said audion to introduce a correction neutralizing the distortion introduced by said element.

2. In a radio receiver comprising a' castype and a detector of the type producing inherent distortion, circuit connections between said audion and detector for impressing upon the input circuit of said amplifier audion the audio frequency-response of said detector, and means impressing a negative bias potential upon said audion of a magnitude suflieient-to bring the operating point of said audion to a pqirved portion of its voltsampere character- 1s 1c. I

3. Method of operating an electrical circuit including a vacuum tube amplifier and a de-' fier effective to amplify a received signal nal energy to produce an audio frequency upon the grid of said audion am 1' caded radio frequency amplifier of the audion tector to neutralize distortion which comprises adjusting said amplifier to operate upon a curved portion of its characteristic curve, detecting the amplified currents by a detector having inherent distortion ahd employing said detected currents to control the adjustment of said amplifier in such a manner as to produce therein a neutralizing distortion.

4. Method of operating an electrical circuit including a vacuum tube amplifier and a detector to neutralize distortion which comprises biasing said amplifier tooperate' upon a curved portion of its characteristic, passing currents through said amplifier, detecting said currents by a detector having inherent distortion, and employing said detected currents to vary the bias of'said amplifier in such a manner as to produce therein a neutralizing distortion.

5. Method of operating a vacuum tube detector to neutralize inherent distortion thereof, which comprises impressing currents upon said detector, detecting said impressed currents, and employing said detected currents to produce a neutralizing distortion in said impressed currents. v

6. The method of automatically controlling the output of an electrical communication system of the type including an audion ampliof the modulated carrier wave ty e and adetector for demodulating the amp ified sig-f nal energy to produce an audio frequency current, which method comprises impressing upon an element of said audion amplifier a direct current otential of such magnitude that said amplifier operates upon a curved portion of its characteristic curve, and va ing the amplification of said audion amp ifier by superposing upon said applied potential a second potential derived from and con trolled in magnitude by rectified signal en- The method of automatically controlling the output of an electrical communica tion system of the type including an audion amplifier e'fi'ective to amplify a received signal of the modulated carrier wavetype and a detector for demodulating the amplified sigcurrent,- which method comprises im ressing v er a direct current bias of such magnitu ethat said amplifier operates upon a curved portion of its characteristic curve, and var in the amplification of said audion amplifier y su erposing upon said applied bias a potentia de- 7 rived from and controlled in magnitude by rectified signal energy.. i

' 8. In an electrical communication'system, the combination with a radio frequency ampl fying audion, a demodulator for the, amphfied output of said audion, and means impressing upon an element of said audion a direct current potential of suchtmagnitude 18! i that said audion operates upon a curved por- 9, in a radio receiver, the combination with:

a radio frequency amplifying audion working into a demodulator, and means im ressing upon the grid of said audion a irect applied potential acurrent potential efiective to place the. operating point of said audion at a curved portion of its characteristic curve, and means for automatically superposing upon said direct 15 current bias a second potential which varies in accordance with the magnitude of rectified signal energy, to vary thereby the amplification of said audion.

In testimony whereof, I aflix my signature.

, STUART BALLANTINE. 

